1. Field of the Invention
The invention generally relates to hard disk file units and more particularly to the configuration of a head disk unit within the hard disk file unit.
2. Description of the Prior Art
Conventional hard disk drives include disks that have recording tracks concentrically spaced on each disk surface. Several of these disks are stacked and mounted on a spindle and subsequently called a disk pack. The disk pack is enclosed in a clean enclosure called the disk drive housing.
As the disks spin at very high speed, read/write elements or heads read information from or record information to the recording tracks on the spinning disks. Each read/write element is associated with a single disk.
Typical head disk assemblies include a read/write element (read/write head) carried on a slider. The slider is suspended from a positioning arm. A servo controlled actuator system connected to the positioning arm electrically moves the slider suspended from the arm across the disk surface from recording track to recording track. The actuator may be either a linear or rotary type. A portion of the positioning arm is flexible and acts as a flexible suspension attached slider and read/write head. A number of such arm/suspension/slider assemblies (hereafter referred to collectively as a head positioning assembly)can move in unison to access multiple disk surfaces. When the disk pack and head positioning assembly are maintained in operational relation to each other in the housing the assembly is known as the head disk assembly.
The rotational speed of the disk pack is limited by such factors as rotational drag, and aerodynamic drag on components. For example, without the head positioning assembly the disks inside the housing spin relatively freely, encountering relatively little air resistance because the rotational air flow is unobstructed. A significant portion of the air near the axis of the disk pack spins in nearly solid body rotation in relation to the disks. Air immediately adjacent to each disk surface is pumped outward by centrifugal forces. There is an accompanying flow in a radial plane between each disk, referred to as return flow, that carries air from the outer perimeter of the disk pack back toward the hub of the spindle.
In some head disk assemblies the spindle hub is vented to facilitate air movement. In these vented assemblies the air introduced at the hub gains angular momentum as it is pumped outward, resulting in a smoother air flow, but at the expense of a greater rotational torque. The work done by the spindle manifests itself as aerodynamic dissipation (heat) inside the head disk assembly and causes the internal temperature of the head disk assembly to rise. This occurs, especially, when the air inside the head disk assembly is recirculated. Therefore, low internal aerodynamic dissipation is desirable to maintain low operational temperatures. Unfortunately, it is difficult to achieve low dissipation.
The resulting rise in internal head disk assembly temperature is undesirable for a number of reasons. One reason is that increased outgassing of materials can later contribute to unfavorable head/disk interactions.
Typical systems that allow air flow through solve the aforementioned temperature problem by constantly filtering and taking in new air. The new air exhausts past components being cooled, such as voice coil motors. The newly introduced air gains angular momentum from the disks, which is subsequently destroyed by radially exhausting the air. Unfortunately, flow-through type systems and the aforementioned method of venting the spindle hub sap energy from the spindle motor.
The head positioning assembly can be a major source of aerodynamic dissipation within the head disk assembly. In conventional disk drives the head positioning assembly partially blocks the air flow circulation inside the disk pack. This partial obstruction leads to an aerodynamic wake adjacent to the trailing edge of the head positioning assembly. The Reynolds number based on the circumferential speed, the distance between the disks, and the kinematic viscosity of the air is such that the flow between the disks is turbulent and thus unsteady. Reynolds numbers are dimensionless numbers corresponding to fluid flow characteristics such as turbulent or laminar flow.
Turbulent air flow causes vibrations in the head positioning assembly, known in the art as "buffeting". In some head disk designs, air flow induced vibrations may exhibit an oscillation that is unstable for small amplitudes, but limited at some finite amplitude, usually by non-linear effects. These aero-elastic vibrations are known in the art as "flutter". Buffeting and flutter disturb the positioning system, causing data errors to occur while reading from or writing to the disks.
A primary disk assembly objective is increasing hard disk storage capacity while maintaining or reducing the drive's volume, i.e. increasing the volumetric storage density. Increasing the volumetric storage density requires using thin disks that are more susceptible to being moved by the air flow. This phenomenon is known as aerodynamic excitation. In the case of thin flexible disks, aerodynamic excitation causes disk bending which can cause servo positioning errors that make it more difficult to increase track density.
Another disk assembly objective is to rotate the disks faster thereby increasing the speed that information can be transferred to/from the disks. The increased rotational speed allows information to be either written to the disks faster or information to be retrieved more quickly from the disks. However, spinning the disks faster intensifies the aerodynamic interference within the disk enclosure.
Low aerodynamic dissipation is even more desirable in limited power applications, e.g. laptop computers. Reducing the aerodynamic dissipation within the hard drive reduces the associated operational motor load so that power consumption decreases, thereby extending laptop battery life.
There is therefore a need for reducing aerodynamic dissipation and aerodynamic interference in a head disk assembly.